Methods and structures for control of microwave propagation



June 28, 1960 J. BAKURA EIAL 2,943,284

METHODS AND STRUCTURES FOR CONTROL OF MICROWAVE PROPAGATION Filed April 8, 1954 I hhhllhllhh 17 4 ,NVENTORS JO E BAKU WILBUR L.. PRITCHARD cllzcuLAiz COAX/AL SECTION ssc'r/o/v B 8% m NEY 2,943,284 Patented June 28, 1960 lVIETI-IODS AND STRUCTURES FOR CONTROL OF MICROWAVE PROPAGATION Joseph Bakura and Wilbur .L., Pritchard, Watertown, Mass., assignors to Raytheon-Company, a corporation of Delaware Filed Apr. 8, 1954, Ser. No. 421,730

2 Claims. (Cl. 333-83) This invention relates to radiant energy, and particutotal phase shift from one end to the other of the cavity. Since the velocity of propagation in a waveguide is different from that of a coaxial transmission line, the

larly to the propagation of microwave energy in a wave confining transmission line leading to a radiating antenna assembly.

The invention consists in the provision of a method of controlling resonant frequency by (a) causing a super-. high frequency wave to travel sequentially along two transmission lines, one circular and the other coaxial, and

(b) adjusting the phase displacement of the wave by mechanically varying the ratio 1 /1 wherein l and 1 respectively, represent the physical lengths of the successively arranged circular and coaxial segments of the transmission line.

The invention also includes certain structural features lending themselves to practice of the control method above outlined. In this connection, an important characteristic of the invention is the utilization of a resonant cavity having a tunable solid dielectric element incorporated therein; tuning being achieved by mechanically varying the electrical length of the dielectric element, while maintaining constancy of mechanical length. Such tuning produces a variation in the /1 ratio above referred to and thereby establishes the desired resonance factor.

Other characteristics of the invention will be better understood upon reference to the following description of the embodiment of the invention illustrated in the accompanying drawings wherein:

Fig. 1 is a longitudinal sectional view of a wave guide and antenna feed assembly showing the invention applied thereto;

Fig. 2 is a view of the cavity;

Fig. 3 is a transverse sectional view along line 3-3' of Fig. 2; and

Fig. 4 is an explanatory diagram.

In these views, reference numeral 11 designates an antenna feed having a resonant cavity structure clamped thereto by screws 12 passing through aligned holes in a base plate 13 and a flange 14, the latter being an integral part of a heater block 15 centrally bored to receive a quartz crystal dielectric element 16 which may be provided with a conductive coating if desired, a silversoldered brass retainer cap 17, a tuning screw 18, a coiled compression spring 19, a spring abutment plate 20, an adjustment locking washer 21, and a protective cap 22. The block 15 has other bores 23 and 24 for reception of heater and thermally responsive elements (not shown) of conventional design, not atfecting or entering into the subject invention.

Dielectric element 16, in conjunction with screw 18, constitutes, in effect, a pair of transmission lines in end to end relationship, and forming a resonant circuit. One portion of the cavity (having the dimension 1 as shown in Fig. 4) is a circular wave guide section and the other (1 in Fig. 4) is a coaxial section. Section 1 need not be circularit could be of any cross-section, so long as it is not electrically conductive. The two sections will become resonant as a cavity when there is a 180 degree oif wave length for the coaxial section is total phase displacement (and the resonant frequency) can be changed by turning screw 18 to vary the relative lengths :of the sections, while keeping the over-all me- Excitation is applied to the chanical length constant.

circular section l 'which generates a TB circular wave guide mode in thissection. A TE coaxial mode is, in

turn, excited in the coaxialsection, The proper mode is selected bychoice of dimensions. .The cutoff wave length of the circular'section is obtained from the ex-- pression 11 2. 1.84 v where D is the diameter of the section and 1.84 is the root of the Bessel function for the TB mode. The cutobtained from the expression where a and b are the radii of the inner and outer conductors and K is a correction factor for various ratios of a/b. It may be noted at this point that, at thetransition from circular to coaxial section, a s'usceptance may be. present which will add a phase shift. However, this shift will be a relatively constant value and does not materially affect the tuning. The rate of tuning and the tuning range are functions of AC AC Ag;, and kg the latter being a function of wave length, derivable as follows:

where C is either C or C: as set forth above. Thus, the tuning rate and range can be controlled'by proper selection of parameters.

This type of cavity has been fabricated and tested with satisfactory results. A tuning range of six percent has been achieved withoutdifiiculty; it is indicated that wider tuning ranges are also feasible. Over the tuning range, extremely little change in Q could be detected. This is of importance since substantially constant Q is desired in most applications. From a mechanical viewpoint, the solid cavity Heifers advantages of (a) reduced size, (b) elimination of humidity and sealing problems, (0) ability to be soldered directly into the 7 wave guide, and (d) low cost and mechanical simplicity.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is, accordingly, desired that the appended claims be given. a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In a resonant cavity structure, in combination, a solid dielectric, tunable, resonant element having a single elongated bore completely internal to said element and extending into said element from one surface only partially through said element, a conductive member extending into the interior of said element along the axis of said bore and adapted to be moved along said bore axis whereby said conductive member divides said element into two effectively different transmission line portions which meet at a plane to which the axis of said bore extending into said element from one surface thereof,

only partially through said element, a conductiv'enie'm ber extending into the interiorof said element a l ong the: axis of said bore and adapted to be moveda'lpng said bore whereby said conductive member dividesjfsaifl fe ment into two portions in end-to-end relationshipfone ofsaid portions comprising, a coaxial"transmis sion'lirie; said portions meeting at a'plane'to which said. axis of said bore is norinal andwhich contains the f'innerriiosltg end of said conductivej nnember, the ratio hetween tlie i s l h r ft a f 9 mini ns. nd h -1m fi quency at which said element can be made to resonate, being dependent upon the extent of the insertion of said conductive member into said bore.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,711 Southworth Sept. 13, 1938 2,417,542 Carter Mar. 18, 1947 2,510,064. Bryan June 6, 1950 2,530,541' Rbth' .l' Nov. 21, 1950 2,562,921 Kandoian Aug. 7, 1951 2,704,830 Rosencrans Marf22, 1955 2,763,843 Cushman Sept. 18, 1956 FOREIGN PATENTS.-

976,767 France Nov. 1, 1950 

